Worm gears are the least common application area for plastic due to a number of limiting factors in worm design. Worm gear sets are composed of a small driving gear which resembles a screw (the worm) mated with a larger take-off gear called the wheel, the teeth of which are curved to conform to the diameter of the worm. Because a throated contact line cannot be molded into plastic gears as readily as in machined metal worms, a simple helical configuration is usually given to the wheel. As a result, the gear load is transmitted across very small contact points between the teeth of the helical wheel and the threads of the worm. This leads to high pressures, surface temperature build-up and excessive wear.
Since the teeth of the helical wheel are weaker than the threads of the worm gear, maximum output torque is limited by the torque capacity of the helical gear. Thus, the load-bearing capacity of plastic worm gear sets is lower than that of other gear types. Plastic Limitations in Worm
Gear Sets • High pressure
• Surface temperature build-up • Excessive wear
Torque Capacity
Helical gears determine torque capacity.
Gears
Due to the dissimilar configuration of the toothed wheel and the screw-like worm, extremely high sliding forces are present during operation. These can increase the strength of the material and promote a higher wear rate. Worm gears generally have very small diameters since they are typically cut or molded as part of a drive shaft. The nature of most applications is such that the shaft is not well-supported at both ends. Overloading and/or stalling torques can cause the shaft to bend axially with a resultant interference in the meshing gears.
Also, because of the small diameter of the worm gear, the tooth size of the wheel is limited. Since the load is concentrated on only a small area of the tooth, there is uneven stress distribution across its width.
Initial lubrication should always be provided and continuous lubrication is suggested wherever possible. Other heat-dissipation methods, should be considered, particularly when external heat can be carried into the gear, as from a motor.
For worm gear calculations, the most liberal safety factors should be applied to allow for stress concentrations due to limited point contact.
Backlash
Also known as “circumferential play” or “tangential clearance,” backlash is generally defined as the distance by which tooth- space exceeds tooth thickness, as measured on the pitch circle. The purpose of backlash is to prevent gears from binding or seizing-up by making simultaneous contact on both sides of meshing teeth.
4-32 • Product Design
Backlash is necessary for several reasons and is influenced by various factors, among them:
• operating temperatures • thermal expansion • mounting tolerances • tooth shape and position • center distance tolerances • tooth size and accuracy
• post-mold dimensional changes • other components of the assembly • runout of shaft bearings
• gear load, speed and running conditions
All of these are interdependent to some degree, but the most critical factor is temperature.
Gears operating under moderate loads and at moderate speeds at room temperature will be less affected by small variations in backlash. At high load, high speed or high temperatures, gears should have both greater tooth accuracy and additional back- lash to compensate for the effect of thermal expansion. As a starting point, here are suggested backlash values for VALOX resin gears operating at room temperature:
Diametral Pitch Pitch Backlash in (mm)
16 0.004 – 0.006 (0.102 – 0.152) 20 0.003 – 0.005 (0.077 – 0.127) 32 0.002 – 0.004 (0.051 – 0.102)
This is only a starting point, since the operating temperature is dependent upon other factors such as gear load, running speed and tooth size. Gears operating at room temperature at high permissible loads and speeds can experience a rise in tooth temperature of 100˚F (38˚C).
Clearly, any method of heat management at the tooth surfaces will decrease the likelihood of gears binding due to insufficient backlash. One method is through continual lubrication. Another is to pair the plastic gear with a metal mate having a higher heat dissipation factor.
Gears
Backlash may be increased by extending the center distance of mating gears. This provides an advantage of more clearance between the outside diameter of one gear and the base diameter of the other, allowing for expansion at high temperatures without radial interference. However, increasing center distance causes the teeth to mesh outside the pitch circle, which can result in greater wear.
It must also be noted that the specified center distance can be affected by other factors. If, for example, other components of the assembly are molded of plastic, their dimensions may change over time due to thermal expansion or post-mold shrinkage from high temperatures. Gear type can also influence center distance, as with bearing runout due to the axial thrust of helical gears. Therefore, providing backlash by means of increased center distance is a method which should be approached with the utmost caution.
In all cases, backlash and center distance measurements and adjustments should be made under actual operating conditions over time using molded prototypes in order to accurately predict gear performance.